5G NR Frequency Raster: NR-ARFCN, the Channel Raster and the Synchronisation Raster (GSCN)
Two rulers over the spectrum: the fine channel raster that positions carriers (NR-ARFCN) and the coarse synchronisation raster that positions SSBs (GSCN) — what each is for, the formulas, and why NR separated them.
The mental model: two rulers
Imagine the whole band drawn on a wall.
- The channel raster is a ruler with fine ticks. A carrier's reference frequency may sit only on a tick. It exists for RF planning: it keeps carriers on a neat grid and gives every allowed frequency a number, the NR-ARFCN.
- The synchronisation raster is a second ruler with far fewer, coarser ticks. An SSB may sit only on those ticks. It exists for the UE doing a blind cell search: instead of scanning every fine tick (thousands per band), the UE only has to try the coarse ones, each identified by a GSCN.
LTE had just one ruler: the sync signals were always at the carrier centre, so "find the carrier" and "find the sync signal" were the same search, at 100 kHz steps. NR splits them, and that split is the whole story of this article.
Ruler 1: the global frequency raster and NR-ARFCN
TS 38.104 §5.4.2 defines a global frequency raster from 0 to 100 GHz. Every point on it has an integer index N_REF (the NR-ARFCN), and the frequency is
F_REF = F_REF-Offs + ΔF_Global × (N_REF − N_REF-Offs)
| Frequency range | ΔF_Global | F_REF-Offs | N_REF-Offs | N_REF range |
|---|---|---|---|---|
| 0 – 3000 MHz | 5 kHz | 0 MHz | 0 | 0 – 599 999 |
| 3000 – 24 250 MHz | 15 kHz | 3000 MHz | 600 000 | 600 000 – 2 016 666 |
| 24 250 – 100 000 MHz | 60 kHz | 24 250.08 MHz | 2 016 667 | 2 016 667 – 3 279 165 |
Example: n78 at 3 500 MHz → N_REF = 600 000 + (3500 − 3000) MHz / 15 kHz = 600 000 + 33 333.3 → nearest valid 633 333 (3 499.995 MHz) or 633 334. NR-ARFCN 632 628 → 3000 + 32 628 × 0.015 = 3 489.42 MHz.
The NR-ARFCN is what you see in RRC (absoluteFrequencyPointA, absoluteFrequencySSB, carrierFreq in measurement objects) and in drive-test logs.
The per-band channel raster
Not every global-raster point is allowed in every band. Each band defines a channel raster ΔF_Raster as a multiple of ΔF_Global (TS 38.101-1 Table 5.4.2.3-1):
- Refarmed sub-3 GHz bands (n1, n3, n7, n8, n20, n28, …): ΔF_Raster = 100 kHz = 20 × ΔF_Global. Same 100 kHz grid as LTE, so an NR carrier can sit exactly where an LTE carrier was — essential for refarming and DSS.
- New NR bands (n77, n78, n79, n41 in part, n257/n258/n260/n261): ΔF_Raster = the SCS itself (15 or 30 kHz in FR1; 60 or 120 kHz in FR2). The carrier grid then lines up with the subcarrier grid, so carriers can be packed without wasted subcarriers.
The RF reference frequency F_REF of a carrier is its centre; from F_REF and N_RB the UE derives Point A.
Ruler 2: the synchronisation raster and GSCN
Because NR lets the SSB sit anywhere on the carrier rather than at its centre (so one carrier can have its SSB placed to suit LTE coexistence, or several carriers can share a search grid), the UE cannot infer the SSB position from the carrier position. If it had to search every channel-raster point for an SSB at 5 kHz steps across 3 GHz, initial cell search would take minutes. So TS 38.104 §5.4.3 defines a sparse set of allowed SSB centre frequencies SS_REF, indexed by the Global Synchronisation Channel Number (GSCN):
| Frequency range | SS_REF | GSCN | GSCN range |
|---|---|---|---|
| 0 – 3000 MHz | N × 1200 kHz + M × 50 kHz, N = 1 … 2499, M ∈ {1, 3, 5} | 3N + (M − 3)/2 | 2 – 7498 |
| 3000 – 24 250 MHz | 3000 MHz + N × 1.44 MHz, N = 0 … 14 756 | 7499 + N | 7499 – 22 255 |
| 24 250 – 100 000 MHz | 24 250.08 MHz + N × 17.28 MHz, N = 0 … 4383 | 22 256 + N | 22 256 – 26 639 |
So the sync-raster step is 1.2 MHz below 3 GHz, 1.44 MHz up to 24.25 GHz, and 17.28 MHz in FR2 — hundreds of times coarser than the channel raster. Per band, TS 38.101-1 Table 5.4.3.3-1 further restricts the allowed GSCNs (a range and sometimes a step of >1).
What the M ∈ {1, 3, 5} offset is for
Below 3 GHz the carrier sits on a 100 kHz raster but the SSB's 20 RBs must line up with the carrier's subcarrier grid, which is at 15 kHz spacing. 1.2 MHz steps alone cannot always achieve that. The ±100 kHz shift (M = 1 or 5 instead of 3) gives the planner a way to land the SSB on a subcarrier boundary of the carrier. Above 3 GHz the channel raster already equals the SCS, so no such offset is needed and the sync raster is a plain 1.44 MHz grid.
Why the SSB does not have to be subcarrier-aligned anyway
Even with the M trick, the SSB's subcarrier 0 may not coincide with a CRB boundary. That residual is exactly what k_SSB in the MIB signals (0–23 subcarriers in FR1, 0–11 in FR2). The UE finds the SSB on the sync raster, reads k_SSB, and shifts to the CRB grid. The raster and k_SSB are two halves of one mechanism.
How the UE actually searches
- For each band it supports, take the allowed GSCN list.
- For each GSCN, tune to SS_REF and try to detect a PSS (3 sequences) at the SSB SCS(s) allowed for that band, listening at least 20 ms.
- On success, detect SSS → PCI, decode PBCH → MIB, read k_SSB and pdcch-ConfigSIB1 → CORESET#0 → SIB1 →
offsetToPointA,absoluteFrequencyPointA, carrier bandwidth.
The number of candidate frequencies is what determines cold-start time. n78 (3.3–3.8 GHz) has roughly 350 GSCN points; scanning at 5 kHz channel-raster steps instead would be 33 000 points.
Why this is better than LTE
| LTE | NR | |
|---|---|---|
| Sync position | fixed at carrier centre | anywhere on the sync raster |
| Search grid | 100 kHz channel raster | sync raster, 1.2 / 1.44 / 17.28 MHz |
| Consequence | small bands, fast search, but SSB placement is rigid | huge bands searched quickly; SSB placement free; several carriers can share one SSB grid; SSB can avoid LTE PSS/SSS in DSS |
The price is the k_SSB / offsetToPointA bookkeeping — cheap for what it buys.
Common confusions
- "GSCN is just another name for ARFCN." Different ruler, different formula, different purpose: ARFCN numbers the channel raster (carriers); GSCN numbers the sync raster (SSBs).
- "The SSB is at the carrier centre." Only by coincidence in NR.
- "
absoluteFrequencySSBis a GSCN." In RRC it is expressed as an NR-ARFCN; the UE converts. GSCN is the search-side index. - "The sync raster is finer than the channel raster." It is much coarser — that is the point.
- "A UE must scan the entire FR1." It scans the GSCNs of the bands it supports, and modern UEs prioritise stored frequencies from previous camping.
Spec pointers
- TS 38.104 §5.4.2 — global frequency raster, NR-ARFCN (Table 5.4.2.1-1); §5.4.3 — sync raster, GSCN (Table 5.4.3.1-1)
- TS 38.101-1 §5.4.2.3 / §5.4.3.3 — per-band channel raster and applicable GSCNs; TS 38.101-2 for FR2
- TS 38.213 §4.1 — cell search, k_SSB
- TS 38.331 — absoluteFrequencySSB, absoluteFrequencyPointA, offsetToPointA
Recap
Two rulers. NR-ARFCN: fine global grid (5 / 15 / 60 kHz steps), restricted per band to a 100 kHz raster (refarmed bands) or the SCS (new bands); it positions carriers. GSCN: coarse grid (1.2 / 1.44 / 17.28 MHz) that positions SSBs so blind search is fast; the M-offset and k_SSB tie the SSB back onto the carrier's subcarrier grid. Separating them is what lets NR put the SSB anywhere and still be found quickly.
Interview questions
The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.
18 questions
- The NR Absolute Radio Frequency Channel Number — an integer that identifies a point on the global frequency raster, from which the RF reference frequency is computed.
- The complete set of frequencies expressible as an NR-ARFCN. It has a granularity of 5 kHz below 3 GHz, 15 kHz from 3 to 24.25 GHz, and 60 kHz above.
- The channel raster is the per-band subset of global raster points on which a carrier may actually be placed. It is coarser than the global raster and is defined band by band.
- A much coarser set of frequencies on which an SSB may be placed, identified by a GSCN. It exists to limit how many positions a UE must search during initial access.
- The Global Synchronization Channel Number — the index of a synchronisation raster point, analogous to NR-ARFCN but for SSB placement.
- LTE placed the synchronisation signals at the centre of the carrier, so finding the carrier and finding sync were the same problem. NR decoupled them so the SSB can sit anywhere in a wide carrier, and the coarse sync raster keeps the search tractable.
- Initial cell search is blind. Every candidate frequency costs time and power, so fewer candidates means faster acquisition — the difference between hundreds and tens of thousands of hypotheses.
- No, and this is the key change from LTE. It can be placed anywhere that coincides with a sync raster point inside the carrier.
- A finer offset within the 1200 kHz step, giving three possible positions per step so the SSB can be placed closer to a desired frequency than the coarse step alone would allow.
- Not to the carrier's own grid. The SSB has its own subcarrier grid, and the offset between them is signalled so the UE can relate the two once it has decoded the MIB.
- Through offsetToPointA in SIB1 and the kSSB offset in the MIB, which together locate Point A relative to the SSB the UE has already found.
- The UE sweeps sync raster points for the band, correlates for PSS, then SSS, derives the physical cell identity, decodes the MIB on PBCH, then finds CORESET#0 and SIB1, and from there the carrier grid.
- Absolute frequency errors scale with carrier frequency, and channel bandwidths are wider higher up, so a fixed fine granularity would be both unnecessary and expensive to search.
- The UE never finds the cell. The carrier may be transmitting correctly, but if the SSB is not on a sync raster point the device has no reason to look there.
- Yes. A wide carrier can carry SSBs for different purposes or cells, which is part of why the decoupling from carrier centre was needed.
- Using the piecewise formula F = F_ref_offset + Δf_global × (N_REF − N_REF_offset), with the offsets and granularity chosen by the frequency range.
- It restricts carrier placement to a manageable set within each band, so a UE scanning a band has a bounded list of centre frequencies rather than every 5 kHz step.
- The channel raster says where the carrier may sit; the synchronisation raster says where the SSB may sit, and it is deliberately coarse so that finding a cell is fast.
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